Improvements in vehicle speed control

ABSTRACT

A speed control system for a vehicle. The speed control system has a torque control for automatically causing application of positive and negative torque to one or more wheels of a vehicle to cause a vehicle to travel in accordance with a target speed value. The system receives information indicative of a gradient of a driving surface over which the vehicle is driving, with the torque control being configured to control the rate of change of the amount of torque applied to the one or more wheels, in order to attempt to maintain the vehicle traveling in accordance with the target speed value, in dependence at least in part on the gradient of the driving surface.

INCORPORATION BY REFERENCE

The content of co-pending UK patent applications GB1214651.0 andGB1202879.1 are hereby incorporated by reference. The content of U.S.Pat. No. 7,349,776 and co-pending international patent applicationsPCT/EP2013/053385 and WO2014/139875 are incorporated herein byreference. The content of UK patent applications GB1111288.5,GB1211910.3 and GB1202427.9 are also incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to a system for controlling the speed of avehicle. In particular, but not exclusively, the invention relates to asystem for controlling the speed of a land-based vehicle which iscapable of driving in a variety of different and extreme terrains andconditions.

BACKGROUND

In known vehicle speed control systems, typically referred to as cruisecontrol systems, the vehicle speed is maintained on-road once set by theuser without further intervention by the user so as to improve thedriving experience for the user by reducing workload. Cruise controlspeed (or cruise set-speed) is settable by the vehicle driver, typicallyby pressing a button when the vehicle is at the desired speed. Plus andminus buttons provide for incremental speed variation while the cruisecontrol is set.

Once the user has selected a speed at which the vehicle is to bemaintained, the vehicle is maintained at that speed for as long as theuser does not apply a brake or, in the case of a vehicle having a manualtransmission, depress a clutch pedal. The cruise control system takesits speed signal from a driveshaft speed sensor or wheel speed sensors.When the brake or the clutch is depressed, the cruise control system isdisabled so that the user can override the cruise control system tochange the vehicle speed without resistance from the system. When thecruise control system is active, if the user depresses the acceleratorpedal the vehicle speed will increase, but once the user removes hisfoot from the accelerator pedal the vehicle reverts to the pre-setcruise speed by coasting.

Such systems are usually operable only above a certain speed, typicallyaround 15-20 kph, and are ideal in circumstances in which the vehicle istravelling in steady traffic conditions, and particularly on highways ormotorways. In congested traffic conditions, however, where vehicle speedtends to vary widely, cruise control systems are ineffective, andespecially where the systems are inoperable because of a minimum speedrequirement. A minimum speed requirement is often imposed on cruisecontrol systems so as to reduce the likelihood of low speed collision,for example when parking. Such systems are therefore ineffective incertain driving conditions (e.g. low speed) and are set to beautomatically disabled in circumstances in which a user may not considerit to be desirable to do so.

More sophisticated cruise control systems are integrated into the enginemanagement system and may include an adaptive functionality which takesinto account the distance to the vehicle in front using a radar-basedsystem. For example, the vehicle may be provided with a forward-lookingradar detection system so that the speed and distance of the vehicle infront is detected and a safe following speed and distance is maintainedautomatically without the need for user input. If the lead vehicle slowsdown, or another object is detected by the radar detection system, thesystem sends a signal to the engine or the braking system to slow thevehicle down accordingly, to maintain a safe following distance.

Known cruise control systems also cancel in the event that a wheel slipevent is detected requiring intervention by a traction control system(TCS) or stability control system (SCS). Accordingly, they are not wellsuited to maintaining vehicle progress when driving in off roadconditions where such events may be relatively common.

Some vehicles are adapted for off-highway use, and it would be desirableto provide low-speed cruise control for such vehicles so as to permitprogress to be maintained over rough terrain. In off-highway conditionscruise control may permit a driver, particularly a novice driver, toconcentrate upon activities such as steering.

It is against this background that the present invention has beenconceived. Embodiments of the invention may provide an apparatus, amethod or a vehicle which addresses the above problems. Other aims andadvantages of the invention will become apparent from the followingdescription, claims and drawings.

It is also known to provide a control system for a motor vehicle forcontrolling one or more vehicle subsystems. U.S. Pat. No. 7,349,776discloses a vehicle control system comprising a plurality of subsystemcontrollers including an engine management system, a transmissioncontroller, a steering controller, a brakes controller and a suspensioncontroller. The subsystem controllers are each operable in a pluralityof subsystem function or configuration modes. The subsystem controllersare connected to a vehicle mode controller which controls the subsystemcontrollers to assume a required function mode so as to provide a numberof driving modes for the vehicle. Each of the driving modes correspondsto a particular driving condition or set of driving conditions, and ineach mode each of the sub-systems is set to the function mode mostappropriate to those conditions. Such conditions are linked to types ofterrain over which the vehicle may be driven such as grass/gravel/snow,mud and ruts, rock crawl, sand and a highway mode known as ‘specialprograms off’ (SPO). The vehicle mode controller may be referred to as aTerrain Response (TR) (RTM) System or controller. The driving modes mayalso be referred to as terrain modes, terrain response modes, or controlmodes.

SUMMARY OF THE INVENTION

In one aspect of the present invention for which protection is soughtthere is provided a speed control system for a vehicle, comprising:

means for automatically causing application of positive and negativetorque to one or more wheels of a vehicle to cause a vehicle to travelin accordance with a target speed value v_target;

means for controlling a rate of change of speed of a vehicle byapplication of positive and negative torque to one or more wheels; and

means for receiving information relating to a gradient of a drivingsurface over which the vehicle is driving,

the means for controlling the rate of change of speed being configuredto control the rate of change of speed in dependence at least in part onthe driving surface gradient.

It is to be understood that in some embodiments the means forcontrolling a rate of change of speed of a vehicle by application ofpositive and negative torque to one or more wheels may be configured toapply positive torque when required, for example when it is desirable tomaintain a current value of vehicle speed and an external force opposesthe maintaining of that speed, or when it is required to accelerate thevehicle, and to apply negative torque when required, for example when itis required to maintain a current speed in the presence of a forceaccelerating the vehicle such as gravity, or decrease vehicle speed. Insome embodiments the means for controlling a rate of change of speed ofa vehicle by application of positive and negative torque to one or morewheels may be configured to apply both positive and negative torque toone or more wheels substantially simultaneously when required.

It will be understood from the following description that a vehiclesuited for off road driving may have a number of terrain response modesin which it can be configured, the terrain response modes eachcorresponding to a vehicle configuration or control strategy suited to aparticular terrain type. The terrain types may include: sand; mud andruts; ice; grass, gravel, snow; wading (water crossing); and a generalmode referred to as special programs off of SPO.

It will also be understood from the following that the terrain responsemode may be set by the driver via an HMI interface that enables thedriver to input the terrain type over which he is driving or intends todrive the vehicle, or alternatively the vehicle may include a pluralityof vehicle parameter sensors, and optionally environmental sensors, anda controller configured to analyze the signals form the sensors, todetermine the terrain type over which the vehicle is being driven, andautomatically configure the vehicle for travel over the prevailingterrain type.

In one aspect of the invention for which protection is sought there isprovided a speed control system for a vehicle, comprising:

torque control means for automatically causing application of positiveand negative torque to one or more wheels of a vehicle to cause avehicle to travel in accordance with a target speed value; and

means for receiving information indicative of a gradient of a drivingsurface over which the vehicle is driving,

the torque control means being configured to control the rate of changeof the amount of torque applied to the one or more wheels, in order toattempt to maintain the vehicle traveling in accordance with the targetspeed value, in dependence at least in part on the gradient of thedriving surface.

This feature has the advantage that the responsiveness of a vehicle todeviations of the actual speed of the vehicle from the target speedvalue may be adjusted in dependence on the driving surface gradient inorder to enhance vehicle composure and driver enjoyment of the vehicle.It is to be understood that, if driving uphill over relatively slipperyterrain, an increased responsiveness to deviations of vehicle speedbelow the target speed reduces the risk that the vehicle fails to makeadequate progress over the terrain, particularly if deviations below thetarget speed (known as ‘undershoot’) occur.

Optionally, the torque control means is configured to attempt to causethe vehicle to travel at a speed substantially equal to the targetspeed, the torque control means being configured to control the rate ofchange of the amount of torque applied to the one or more wheels, inorder to attempt to maintain the vehicle traveling substantially at thetarget speed value, in dependence at least in part on the gradient ofthe driving surface.

Optionally, the torque control means is configured wherein when actualvehicle speed is less than the target speed value and the informationindicative of surface gradient indicates the vehicle is travelinguphill, the torque control means attempts to cause the vehicle toaccelerate towards the target speed value at a rate that is higher thanwhen driving on a substantially horizontal surface.

In other words, when the vehicle is travelling uphill, i.e. on a drivingsurface having a positive gradient relative to a substantiallyhorizontal driving surface, the torque control means may be configuredto attempt to cause the vehicle to accelerate towards the target speedvalue when speed is below the target speed value at a rate that ishigher than when driving on a substantially horizontal surface. That is,the torque control means may be configured to attempt to cause thevehicle to accelerate towards the target speed value when speed is belowthe target speed value at a more aggressive rate than when driving on asubstantially horizontal surface.

Optionally, the torque control means is configured wherein when actualvehicle speed is less than the target speed value, the torque controlmeans attempts to cause the vehicle to accelerate towards the targetspeed value at a rate that is higher for higher values of uphill drivingsurface gradient.

Thus, the torque control means may attempt to increase the amount oftorque at a rate that increases with increasing positive (uphill)driving surface gradient. That is, the torque control means may attemptto increase the amount of torque at a rate that is more aggressive withincreasing gradient steepness.

Alternatively, the torque control means is configured wherein whenactual vehicle speed is less than the target speed value and theinformation indicative of surface gradient indicates the vehicle istraveling uphill, the torque control means attempts to cause the vehicleto accelerate towards the target speed value at a rate that is lowerthan when driving on a substantially horizontal surface.

In other words, when the vehicle is travelling uphill, i.e. on a drivingsurface having a positive gradient relative to a substantiallyhorizontal driving surface, the torque control means may be configuredto attempt to cause the vehicle to accelerate towards the target speedvalue when speed is below the target speed value at a rate that is lessthan the corresponding rate when driving on a substantially horizontalsurface. That is, the torque control means may be configured to attemptto cause the vehicle to accelerate towards the target speed value whenspeed is below the target speed value at a less aggressive rate thanwhen driving on a substantially horizontal surface, for given values ofactual vehicle speed and target speed.

Optionally, the torque control means is configured wherein when actualvehicle speed is less than the target speed value, the torque controlmeans attempts to cause the vehicle to accelerate towards the targetspeed value at a rate that is increasingly lower for increasingly highervalues of uphill driving surface gradient.

Thus, the torque control means may attempt to increase the amount oftorque at a rate that decreases with increasing positive (uphill)driving surface gradient. That is, the torque control means may attemptto increase the amount of torque at a rate that is progressively lessaggressive with progressively increasing gradient steepness.

As discussed below, the control system may determine whether to employincreasingly aggressive rates of acceleration with increasing steepnessof the driving surface, or decreasingly aggressive rates of accelerationwith increasing steepness, in further dependence at least in part on theidentity of a driving mode in which the vehicle is operating. In someembodiments the control system may make this determination in dependenceat least in part on the nature of the terrain over which the vehicle isdriving. In some embodiments the driving mode may be indicative of thenature of the terrain over which the vehicle is driving.

Optionally, the torque control means is configured wherein when actualvehicle speed is greater than the target speed value, the torque controlmeans causes a reduction in torque applied to the one or more wheels inorder to attempt to cause the vehicle to travel at a speed substantiallyequal to the target speed at a rate that is lower for a given deviationin speed above the target speed than in the case of a correspondingdeviation below the target speed value.

Thus, the control system may respond to deviations above the targetspeed (referred to as ‘overshoot’) less aggressively than deviationsbelow the target speed.

The torque control means may be configured wherein when actual vehiclespeed is greater than the target speed value and the vehicle istraveling uphill, the torque control means causes a reduction in torqueapplied to the one or more wheels in order to attempt to cause thevehicle to travel at a speed substantially equal to the target speed ata rate that is substantially equal to or lower than in the case of acorresponding deviation in vehicle speed above the target speed whiletraveling over a substantially horizontal driving surface.

Optionally, the torque control means is configured wherein when actualvehicle speed is greater than the target speed value and the vehicle istravelling downhill, the torque control means causes a reduction intorque applied to the one or more wheels in order to attempt to causethe vehicle to travel at a speed substantially equal to the target speedat a rate that is substantially equal to or lower than in the case of acorresponding deviation in vehicle speed above the target speed whiletraveling over a substantially horizontal driving surface.

Thus, the control system may respond to vehicle speed deviations abovethe target speed (referred to as ‘overshoot’) less aggressively whengoing uphill and/or downhill relative to driving over substantiallyhorizontal surfaces. Alternatively the control system may respond moreaggressively. Other arrangements may be useful.

Optionally, the torque control means is configured wherein when actualvehicle speed is greater than the target speed value and the vehicle istravelling downhill, the torque control means causes a reduction intorque applied to the one or more wheels, in order to attempt to causethe vehicle to travel at a speed substantially equal to the targetspeed, at a rate that is greater than in the case of a correspondingdeviation in vehicle speed above the target speed while traveling over asubstantially horizontal driving surface.

Thus, the control system may attempt to correct vehicle speed moreaggressively in the event of overshoot when travelling downhill due tothe effect of gravity in tending to promote overshoot. Accordingly, theamount of any further increase in vehicle speed may be reduced due tothe relatively aggressive response.

The control system may be configured to cause the vehicle to acceleratefrom a first speed to the target speed value, where the first speed isless than the target speed value, at least in part according to storeddata in respect of target rate of acceleration as a function of speed,wherein the value of target rate of acceleration according to which thevehicle is caused to accelerate is determined in further dependence atleast in part on the driving surface gradient.

The control system may determine the target rate of acceleration independence at least in part on the difference between the currentvehicle speed and target vehicle speed.

The control system may be configured to cause a vehicle to deceleratefrom a second speed to the target speed value, where the second speed isgreater than the target speed value, according to stored data in respectof target rate of deceleration as a function of speed, wherein thetarget rate of deceleration according to which the vehicle is caused todecelerate is determined in further dependence at least in part on thedriving surface gradient.

The control system may determine the target rate of deceleration independence at least in part on the difference between the currentvehicle speed and target vehicle speed.

The control system may be configured to control a rate of change ofvehicle speed towards the target speed iteratively by causing thevehicle to attempt to achieve an intermediate instant target speed, thevalue of intermediate instant target speed and therefore vehicle speedbeing caused to change in an iterative manner towards the target speedvalue at a required rate.

The control system may be operable to control a rate of change ofvehicle speed so as not to exceed a prescribed jerk value.

Optionally, the prescribed jerk value is set in dependence on thegradient of the driving surface.

Optionally, the prescribed jerk value during a decrease in vehicle speedtowards the target speed is higher for lower values of positive drivingsurface gradient and lower for higher values of positive driving surfacegradient.

Alternatively, in some embodiments the prescribed jerk value during adecrease in vehicle speed towards the target speed may be lower forlower values of positive driving surface gradient and higher for highervalues of positive driving surface gradient.

Optionally, the prescribed jerk value during an increase in vehiclespeed is higher for higher values of driving surface gradient and lowerfor lower values of driving surface gradient.

Optionally, the torque control means is configured to control the rateof change of the amount of torque applied to the one or more wheels, inorder to attempt to maintain the vehicle traveling in accordance withthe target speed value, in further dependence at least in part on theidentity of a driving mode in which the vehicle is operating.

Optionally, the driving mode is one of a plurality of driving modes inwhich each one of a plurality of vehicle subsystems is caused to operatein one of a plurality of configuration modes of that subsystem, thesubsystem configuration mode being determined in dependence on theselected driving mode.

The control system may be configured wherein the torque control means isoperable to control the rate of change of the amount of torque appliedto the one or more wheels, in order to attempt to maintain the vehicletraveling in accordance with the target speed value, in dependence atleast in part on the information indicative of driving surface gradientonly if the vehicle is operating in a driving mode that is a member of apredetermined group of one or more of the plurality of driving modes.

The control system may be configured wherein when actual vehicle speedis less than the target speed value and the information indicative ofsurface gradient indicates the vehicle is traveling uphill, the torquecontrol means attempts to cause the vehicle to accelerate towards thetarget speed value at a rate that is lower than when driving on asubstantially horizontal surface if the vehicle is operating in adriving mode that is a member of a first group of one or more of thedriving modes and is not a driving mode that is not a member of thefirst group.

Optionally, the first group of driving modes comprises at least onedriving mode adapted for driving on a driving surface of relatively lowsurface coefficient of friction.

Optionally, the first group of driving modes comprises at least onedriving mode adapted for driving on a driving surface of relatively lowsurface coefficient of friction excluding a mode adapted for driving onsand.

It is to be understood that sand represents a surface that ischaracterized by relatively high drag and relatively highlydeformability as well as relatively low surface coefficient of friction.Accordingly, when driving uphill over sand and undershoot occurs, it isadvisable to respond with a relatively rapid, i.e. aggressive,increasing in torque applied to the one or more wheels, in order toreduce the risk of the vehicle becoming bogged down or otherwiseimmobilized. Slip of wheels when travelling over sand can be effectivein enhancing traction by enabling the wheels to ‘excavate’ sand,compounding the material to improve tractive force.

In contrast, when driving over a surface of relatively low surfacecoefficient other than sand, such as surfaces that are of relatively lowdrag, a less aggressive increase in torque may be advisable whenundershoot occurs in order to reduce the risk of excessive wheel spinand potential resultant loss of traction. Furthermore, undesirablemodification of the driving surface can occur as a consequence ofexcessive wheel slip. In the case of a grassy surface, being typicallyof relatively low surface coefficient of friction, modification of thesurface can result in the exposure of a surface of even lower surfacecoefficient of friction such as mud, making progress over the surfaceeven more difficult.

Optionally, the first group of driving modes comprises at least onedriving mode adapted for driving on at least one of a snowy surface, anicy surface, grass, gravel, snow and mud.

The first group of driving modes may include a snow and ice mode, agrass/gravel/snow mode and/or a mud/ruts mode. Other modes may be usefulin addition or instead.

Optionally, the subsystems include at least one of a powertrainsubsystem, a brakes subsystem and a suspension subsystem.

Optionally, the torque control means comprises an electric controllerconfigured to communicate with a powertrain controller and a brakescontroller.

Optionally, the electric controller further comprises the means forreceiving information indicative of the gradient of the driving surface.

Optionally, the means for receiving indicative of the gradient of thedriving surface comprises an electrical input for receiving anelectrical signal indicative of the gradient of the driving surface.

A speed control system as described above, wherein:

said torque control means comprises an electronic processor having anelectrical input for receiving a signal proving said informationindicative of a gradient of a driving surface over which the vehicle isdriving; and

an electronic memory device electrically coupled to the electronicprocessor and having instructions stored therein,

wherein the processor is configured to access the memory device andexecute the instructions stored therein such that it is operable to:

cause application of positive and negative torque to one or more wheelsof a vehicle to cause a vehicle to travel in accordance with a targetspeed value, and

control the rate of change of the amount of torque applied to the one ormore wheels, in order to attempt to maintain the vehicle traveling inaccordance with the target speed value, in dependence at least in parton the signal providing said information indicative of a gradient of adriving surface.

In a further aspect of the invention for which protection is soughtthere is provided a vehicle comprising a control system according to anypreceding aspect.

In one aspect of the invention for which protection is sought there isprovided a method of controlling a vehicle implemented by means of acontrol system, comprising:

automatically causing application of positive and negative torque to oneor more wheels of a vehicle to cause a vehicle to travel in accordancewith a target speed value; and

receiving information indicative of a gradient of a driving surface overwhich the vehicle is driving,

the method comprising controlling the rate of change of the amount oftorque applied to the one or more wheels, in order to attempt tomaintain the vehicle traveling in accordance with the target speedvalue, in dependence at least in part on the gradient of the drivingsurface.

In one aspect of the invention for which protection is sought there isprovided a speed control system for a vehicle, comprising:

means for automatically causing application of positive and negativetorque to one or more wheels of a vehicle to cause a vehicle to travelin accordance with a target speed value;

means for controlling a rate of change of speed of a vehicle byapplication of positive and negative torque to one or more wheels; and

means for receiving information relating to the gradient of a drivingsurface and the identity of a terrain response mode in which the vehicleis configured,

the means for controlling the rate of change of speed being configuredto control the rate of change of speed in dependence at least in part onsaid information relating to the gradient of a driving surface and theidentity of a terrain response mode in which the vehicle is configured.

Optionally, the control system may receive information relating to theamount of drag imposed on the vehicle by a driving surface and controlthe rate of change of speed in further dependence at least in part onsaid information.

It is to be understood that the information relating to one or more of aterrain response mode and an amount of drag imposed may be informationor data indicative of one or more of a terrain response mode and anamount of drag.

Some embodiments of the present invention have the advantage that aspeed control system may be configured to control application of torqueto one or more wheels of a vehicle travelling over terrain that imposesa relatively large amount of drag on a vehicle in a different manner toa vehicle travelling over terrain that imposes a relatively small amountof drag on a vehicle. As a consequence, in some embodiments a risk thata vehicle fails to make adequate progress over terrain of relativelyhigh drag may be reduced. Furthermore, vehicle composure may beenhanced, because vehicle performance and handling characteristics areaffected substantially by the amount of drag imposed on a vehicle. Bytaking account of the amount of drag, or the selected terrain mode, whencontrolling the rate of change of speed, a vehicle may be caused tooperate in a more comfortable and predictable manner.

Some embodiments of the invention relate to travel over relatively highdrag surfaces such as sand. It is to be understood that in order tomaintain vehicle progress when relatively high drag forces are imposedon a vehicle, a relatively high rate of acceleration may be requiredcompared with that required in the case of relatively low dragconditions, especially when attempting to negotiate an incline. Forexample, if a vehicle is travelling over relatively flat, horizontalterrain that imposes a relatively large amount of drag on the vehicle,such as over sand, and the vehicle begins to ascend an incline such as aside of a sand dune, a rate of deceleration of the vehicle due to theeffect of gravity as a consequence of the incline may be relatively highunless an increase in powertrain torque takes place. Under suchcircumstances, when vehicle speed begins to fall below the target speedas the vehicle begins to climb the dune, the control system may cause apowertrain of a vehicle to impose a relatively high rate of accelerationon the vehicle in order to prevent the vehicle from failing to makeadequate progress over the dune. Such circumstances may also occur whenascending a muddy hill where a relatively large amount of wheel slipoccurs at the vehicle wheels.

Similarly, when a vehicle crests a dune and begins to descend a dune,application of negative torque to one or more wheels in order preventover-shoot or over-run of a target speed may be required in somecircumstances. However, application of brake torque when travelling overrelatively high drag surfaces such as sand may have the effect ofabruptly arresting vehicle progress and causing a loss of vehiclecomposure, particularly when descending an incline. In the case of sandyterrain at least, a vehicle may become immobilized in some cases due toone or more wheels sinking into the surface of the terrain when negativetorque is applied to a wheel. Accordingly, in order to reduce vehiclespeed when descending an incline, the control system may be configuredto decrease an amount of brake torque applied to one or more wheels at agiven moment in time relative to that which would be applied whentravelling over a different surface such as dry asphalt. In someembodiments, substantially no brake torque may be applied. Rather, adrag force on the vehicle due to the terrain may be employed to causedeceleration. Positive drive torque may be maintained, of reducedamount, in order to cause deceleration without applying brake torque, insome embodiments.

In an aspect of the invention for which protection is sought there isprovided a carrier medium carrying a computer readable code forcontrolling a vehicle to carry out the method of another aspect.

In an aspect of the invention for which protection is sought there isprovided a computer program product executable on a processor so as toimplement the method of another aspect.

In an aspect of the invention for which protection is sought there isprovided a computer readable medium loaded with the computer programproduct of another aspect.

In an aspect of the invention for which protection is sought there isprovided a processor arranged to implement the method of another aspect,or the computer program product of another aspect.

Within the scope of this application it is envisaged that the variousaspects, embodiments, examples and alternatives, and in particular thefeatures thereof, may be taken independently or in any combinationthereof. For example, features disclosed in connection with oneembodiment are applicable to all embodiments, unless such features areincompatible.

BRIEF DESCRIPTION OF DRAWINGS

The present invention will now be described, by way of example only,with reference to the accompanying drawings, in which:

FIG. 1 is a schematic illustration of a vehicle according to anembodiment of the invention in plan view;

FIG. 2 shows the vehicle of FIG. 1 in side view;

FIG. 3 is a high level schematic diagram of an embodiment of the vehiclespeed control system of the present invention, including a cruisecontrol system and a low-speed progress control system;

FIG. 4 is a schematic diagram of further features of the vehicle speedcontrol system of FIG. 3;

FIG. 5 illustrates a steering wheel and brake and accelerator pedals ofa vehicle according to an embodiment of the present invention;

FIG. 6 is a schematic illustration of controller gain profiles employedfor a sand terrain response mode when traveling over driving surfaces ofpositive gradient (uphill, profile A), substantially no gradient (i.e.substantially horizontal, profile B) and negative gradient (downhill,profile B); and

FIG. 7 is a schematic illustration of controller gain profiles employedfor a GGS (grass/gravel/snow) terrain response mode when traveling overdriving surfaces of positive gradient (uphill, profile A), substantiallyno gradient (i.e. substantially horizontal, profile B) and negativegradient (downhill, profile B).

DETAILED DESCRIPTION

References herein to a block such as a function block are to beunderstood to include reference to software code for performing thefunction or action specified which may be an output that is providedresponsive to one or more inputs. The code may be in the form of asoftware routine or function called by a main computer program, or maybe code forming part of a flow of code not being a separate routine orfunction. Reference to function block is made for ease of explanation ofthe manner of operation of embodiments of the present invention.

FIG. 1 shows a vehicle 100 according to an embodiment of the presentinvention. The vehicle 100 has a powertrain 129 that includes an engine121 that is connected to a driveline 130 having an automatictransmission 124. It is to be understood that embodiments of the presentinvention are also suitable for use in vehicles with manualtransmissions, continuously variable transmissions or any other suitabletransmission.

In the embodiment of FIG. 1 the transmission 124 may be set to one of aplurality of transmission operating modes, being a park mode, a reversemode, a neutral mode, a drive mode or a sport mode, by means of atransmission mode selector dial 124S. The selector dial 124S provides anoutput signal to a powertrain controller 11 in response to which thepowertrain controller 11 causes the transmission 124 to operate inaccordance with the selected transmission mode.

The driveline 130 is arranged to drive a pair of front vehicle wheels111,112 by means of a front differential 137 and a pair of front driveshafts 118. The driveline 130 also comprises an auxiliary drivelineportion 131 arranged to drive a pair of rear wheels 114, 115 by means ofan auxiliary driveshaft or prop-shaft 132, a rear differential 135 and apair of rear driveshafts 139.

Embodiments of the invention are suitable for use with vehicles in whichthe transmission is arranged to drive only a pair of front wheels oronly a pair of rear wheels (i.e. front wheel drive vehicles or rearwheel drive vehicles) or selectable two wheel drive/four wheel drivevehicles. In the embodiment of FIG. 1 the transmission 124 is releasablyconnectable to the auxiliary driveline portion 131 by means of a powertransfer unit (PTU) 131P, allowing operation in a two wheel drive modeor a four wheel drive mode. It is to be understood that embodiments ofthe invention may be suitable for vehicles having more than four wheelsor where only two wheels are driven, for example two wheels of a threewheeled vehicle or four wheeled vehicle or a vehicle with more than fourwheels.

A control system for the vehicle engine 121 includes a centralcontroller 10, referred to as a vehicle control unit (VCU) 10, thepowertrain controller 11, a brake controller 13 (an anti-lock brakingsystem (ABS) controller) and a steering controller 170C. The ABScontroller 13 forms part of a braking system 22 (FIG. 3). The VCU 10receives and outputs a plurality of signals to and from various sensorsand subsystems (not shown) provided on the vehicle. The VCU 10 includesa low-speed progress (LSP) control system 12 shown in FIG. 3, astability control system (SCS) 14, a cruise control system 16 and a hilldescent control (HDC) system 12HD. The SCS 14 improves the safety of thevehicle 100 by detecting and managing loss of traction. When a reductionin traction or steering control is detected, the SCS 14 is operableautomatically to command the ABS controller 13 to apply one or morebrakes of the vehicle to help to steer the vehicle 100 in the directionthe user wishes to travel. In the embodiment shown the SCS 14 isimplemented by the VCU 10. In some alternative embodiments the SCS 14may be implemented by the ABS controller 13.

Although not shown in detail in FIG. 3, the VCU 10 further includes aTraction Control (TC) function block. The TC function block isimplemented in software code run by a computing device of the VCU 10.The ABS controller 13 and TC function block provide outputs indicativeof, for example, TC activity, ABS activity, brake interventions onindividual wheels and engine torque requests from the VCU 10 to theengine 121 in the event a wheel slip event occurs. Each of theaforementioned events indicate that a wheel slip event has occurred. Insome embodiments the ABS controller 13 implements the TC function block.Other vehicle sub-systems such as a roll stability control system or thelike may also be included.

As noted above the vehicle 100 also includes a cruise control system 16which is operable to automatically maintain vehicle speed at a selectedspeed when the vehicle is travelling at speeds in excess of 25 kph. Thecruise control system 16 is provided with a cruise control HMI (humanmachine interface) 18 by which means the user can input a target vehiclespeed to the cruise control system 16 in a known manner. In oneembodiment of the invention, cruise control system input controls aremounted to a steering wheel 171 (FIG. 5). The cruise control system 16may be switched on by pressing a cruise control system selector button176. When the cruise control system 16 is switched on, depression of a‘set-speed’ control 173 sets the current value of a cruise controlset-speed parameter, cruise_set-speed to the current vehicle speed.Depression of a ‘+’ button 174 allows the value of cruise_set-speed tobe increased while depression of a ‘−’ button 175 allows the value ofcruise_set-speed to be decreased. A resume button 173R is provided thatis operable to control the cruise control system 16 to resume speedcontrol at the instant value of cruise_set-speed following driverover-ride. It is to be understood that known on-highway cruise controlsystems including the present system 16 are configured so that, in theevent that the user depresses the brake or, in the case of vehicles witha manual transmission, a clutch pedal, control of vehicle speed by thecruise control system 16 is cancelled and the vehicle 100 reverts to amanual mode of operation which requires accelerator or brake pedal inputby a user in order to maintain vehicle speed. In addition, detection ofa wheel slip event, as may be initiated by a loss of traction, also hasthe effect of cancelling control of vehicle speed by the cruise controlsystem 16. Speed control by the system 16 is resumed if the driversubsequently depresses the resume button 173R.

The cruise control system 16 monitors vehicle speed and any deviationfrom the target vehicle speed is adjusted automatically so that thevehicle speed is maintained at a substantially constant value, typicallyin excess of 25 kph. In other words, the cruise control system isineffective at speeds lower than 25 kph. The cruise control HMI 18 mayalso be configured to provide an alert to the user about the status ofthe cruise control system 16 via a visual display of the HMI 18. In thepresent embodiment the cruise control system 16 is configured to allowthe value of cruise_set-speed to be set to any value in the range 25-150kph.

The LSP control system 12 also provides a speed-based control system forthe user which enables the user to select a very low target speed atwhich the vehicle can progress without any pedal inputs being requiredby the user to maintain vehicle speed. Low-speed speed control (orprogress control) functionality is not provided by the on-highway cruisecontrol system 16 which operates only at speeds above 25 kph.

In the present embodiment, the LSP control system 12 is activated bypressing a HDC system selector button 177 mounted on steering wheel 171for less than a prescribed time period (in the present embodiment theprescribed time period is 3 s although other values are also useful),and subsequently pressing the ‘set +’ button 174. In some embodiments adedicated LSP control system selector button is mounted on the steeringwheel 171, by means of which the LSP control system 12 is activated. Thesystem 12 is operable to apply selective powertrain, traction controland braking actions to one or more wheels of the vehicle 100,collectively or individually.

The LSP control system 12 is configured to allow a user to input adesired value of set-speed parameter, user set-speed to the LSP controlsystem 12 via a low-speed progress control HMI (LSP HMI) 20 (FIG. 1,FIG. 3) which shares certain input buttons 173-175 with the cruisecontrol system 16 and HDC control system 12HD. Provided the vehiclespeed is within the allowable range of operation of the LSP controlsystem 12 (which is the range from 2 to 30 kph in the present embodimentalthough other ranges are also useful) and no other constraint onvehicle speed exists while under the control of the LSP control system12, the LSP control system 12 controls vehicle speed in accordance witha LSP control system set-speed value LSP_set-speed which is setsubstantially equal to user_set-speed. Unlike the cruise control system16, the LSP control system 12 is configured to operate independently ofthe occurrence of a traction event. That is, the LSP control system 12does not cancel speed control upon detection of wheel slip. Rather, theLSP control system 12 actively manages vehicle behavior when slip isdetected.

The LSP control HMI 20 is provided in the vehicle cabin so as to bereadily accessible to the user. The user of the vehicle 100 is able toinput to the LSP control system 12, via the LSP HMI 20, an indication ofthe speed at which the user desires the vehicle to travel (referred toas “the target speed”) by means of the ‘set-speed’ button 173 and the‘+’/‘−’ buttons 174, 175 in a similar manner to the cruise controlsystem 16. The LSP HMI 20 also includes a visual display by means ofwhich information and guidance can be provided to the user about thestatus of the LSP control system 12.

The LSP control system 12 receives an input from the ABS controller 13of the braking system 22 of the vehicle indicative of the extent towhich the user has applied braking by means of the brake pedal 163. TheLSP control system 12 also receives an input from an accelerator pedal161 indicative of the extent to which the user has depressed theaccelerator pedal 161, and an input from the transmission or gearbox124. This latter input may include signals representative of, forexample, the speed of an output shaft of the gearbox 124, an amount oftorque converter slip and a gear ratio request. Other inputs to the LSPcontrol system 12 include an input from the cruise control HMI 18 whichis representative of the status (ON/OFF) of the cruise control system16, an input from the LSP control HMI 20, and an input from a gradientsensor 45 indicative of the gradient of the driving surface over whichthe vehicle 100 is driving. In the present embodiment the gradientsensor is a gyroscopic sensor. In some alternative embodiments the LSPcontrol system 12 receives a signal indicative of driving surfacegradient from another controller such as the ABS controller 13. The ABScontroller 13 may determine gradient based on a plurality of inputs,optionally based at least in part on signals indicative of vehiclelongitudinal and lateral acceleration and a signal indicative of vehiclereference speed (v_actual) being a signal indicative of actual vehiclespeed over ground. Methods for the calculation of vehicle referencespeed based for example on vehicle wheel speeds are well known. Forexample in some known vehicles the vehicle reference speed may bedetermined to be the speed of the second slowest turning wheel, or theaverage speed of all the wheels. Other ways of calculating vehiclereference speed may be useful in some embodiments, including by means ofa camera device or radar sensor.

When the HDC system 12HD is active, the system 12HD controls the brakingsystem 22 in order to limit vehicle speed to a value corresponding tothat of a HDC set-speed parameter HDC_set-speed which may be set by auser. The HDC set-speed parameter may also be referred to as an HDCtarget speed. Provided the user does not override the HDC system 12HD bydepressing the accelerator pedal 161 when the HDC system 12HD is active,the HDC system 12HD controls the braking system 22 (FIG. 3) to preventvehicle speed from exceeding HDC_set-speed. In the present embodimentthe HDC system 12HD is not operable to apply positive drive torque.Rather, the HDC system 12HD is only operable to cause negative braketorque to be applied, via the braking system 22.

A HDC system HMI 20HD is provided by means of which a user may controlthe HDC system 12HD, including setting the value of HDC_set-speed. TheHDC system is activated by depressing the HDC selector button 177 formore than the prescribed period (3 s in the present embodiment as notedabove).

As noted above, the HDC system 12HD is operable to allow a user to set avalue of HDC set-speed parameter HDC_set-speed and to adjust the valueof HDC_set-speed using the same controls as the cruise control system 16and LSP control system 12. Thus, in the present embodiment, when the HDCsystem 12HD is controlling vehicle speed, the HDC system set-speed maybe increased, decreased or set to an instant speed of the vehicle in asimilar manner to the set-speed of the cruise control system 16 and LSPcontrol system, using the same control buttons 173, 173R, 174, 175. TheHDC system 12HD is operable to allow the value of HDC_set-speed to beset to any value in the range from 2-30 kph.

If the HDC system 12HD is selected when the vehicle 100 is travelling ata speed of 50 kph or less and no other speed control system is inoperation, the HDC system 12HD sets the value of HDC_set-speed to avalue selected from a look-up table. The value output by the look-uptable is determined in dependence on the identity of the currentlyselected transmission gear, the currently selected PTU gear ratio(Hi/LO) and the currently selected driving mode. The HDC system 12HDthen causes the powertrain 129 and/or braking system 22 (via signal 42,FIG. 4) to slow the vehicle 100 to the HDC system set-speed provided thedriver does not override the HDC system 12HD by depressing theaccelerator pedal 161. It is to be understood that the HDC system 12HDmay cause the powertrain 129 to apply negative torque to one or morewheels, for example by engine over-run braking, but cannot cause thepowertrain 129 to apply a positive torque to a wheel.

If actual vehicle speed v_actual exceeds the set-speed valueHDC_set-speed, the HDC system 12HD is configured to slow the vehicle 100to the set-speed value at a deceleration rate not exceeding a maximumallowable rate. The rate is set as 1.25 ms-2 in the present embodiment,however other values are also useful. If the user subsequently pressesthe ‘set-speed’ button 173 the HDC system 12HD sets the value ofHDC_set-speed to the instant vehicle speed provided the instant speed is30 kph or less.

If the HDC system 12HD is selected (by depressing the HDC selectorbutton 177 for more than the prescribed period when the HDC system 12HDand LSP control system 12 are switched off) and the vehicle 100 istravelling at a speed exceeding 50 kph, the HDC system 12HD ignores therequest and provides an indication to the user that the request has beenignored.

It is to be understood that the VCU 10 is configured to implement aknown Terrain Response (TR) (RTM) System of the kind described above inwhich the VCU 10 controls settings of one or more vehicle systems orsub-systems such as the powertrain controller 11 in dependence on aselected driving mode. The driving mode may be selected by a user bymeans of a driving mode selector 141S (FIG. 1). The driving modes mayalso be referred to as terrain modes, terrain response (TR) modes, orcontrol modes.

In the embodiment of FIG. 1 four driving modes are provided: an‘on-highway’ driving mode suitable for driving on a relatively hard,smooth driving surface where a relatively high surface coefficient offriction exists between the driving surface and wheels of the vehicle; a‘sand’ driving mode suitable for driving over sandy terrain, beingterrain characterized at least in part by relatively high drag,relatively high deformability or compliance and relatively low surfacecoefficient of friction; a ‘grass, gravel or snow’ (GGS) driving modesuitable for driving over grass, gravel or snow, being relativelyslippery surfaces (i.e. having a relatively low coefficient of frictionbetween surface and wheel and, typically, lower drag than sand); a ‘rockcrawl’ (RC) driving mode suitable for driving slowly over a rockysurface; and a ‘mud and ruts’ (MR) driving mode suitable for driving inmuddy, rutted terrain. Other driving modes may be provided in additionor instead. In the present embodiment the selector 141S also allows auser to select an ‘automatic driving mode selection condition’ in whichthe VCU 10 selects automatically the most appropriate driving mode asdescribed in more detail below. The on-highway driving mode may bereferred to as a ‘special programs off’ (SPO) mode in some embodimentssince it corresponds to a standard or default driving mode, and is notrequired to take account of special factors such as relatively lowsurface coefficient of friction, or surfaces of high roughness.

In some embodiments, including the present embodiment, the LSP controlsystem 12 may be in either one of an active condition, a standbycondition and an ‘off’ condition at a given moment in time. In theactive condition, the LSP control system 12 actively manages vehiclespeed by controlling powertrain torque and braking system torque. In thestandby condition, the LSP control system 12 does not control vehiclespeed until a user presses the resume button 173R or the ‘set speed’button 173. In the off condition the LSP control system 12 is notresponsive to input controls.

In the present embodiment the LSP control system 12 is also operable toassume an intermediate mode or condition similar to that of the activemode but in which the LSP control system 12 is prevented from commandingthe application of positive drive torque to one or more wheels of thevehicle 100 by the powertrain 129. Thus, only braking torque may beapplied, by means of the braking system 22 and/or powertrain 129. In thepresent embodiment, the intermediate mode is implemented by causing theHDC control system 12HD to control vehicle speed, with the valueHDC_set-speed set substantially equal to LSP_set-speed. Otherarrangements are also useful.

With the LSP control system 12 in the active condition, the user mayincrease or decrease the vehicle set-speed by means of the ‘+’ and ‘−’buttons 174, 175. In addition, the user may also increase or decreasethe vehicle set-speed by lightly pressing the accelerator or brakepedals 161, 163 respectively. In some embodiments, with the LSP controlsystem 12 in the active condition the ‘+’ and ‘−’ buttons 174, 175 aredisabled such that adjustment of the value of LSP_set-speed can only bemade by means of the accelerator and brake pedals 161, 163. This latterfeature may prevent unintentional changes in set-speed from occurring,for example due to accidental pressing of one of the ‘+’ or ‘−’ buttons174, 175. Accidental pressing may occur for example when negotiatingdifficult terrain where relatively large and frequent changes insteering angle may be required. Other arrangements are also useful.

It is to be understood that in the present embodiment the LSP controlsystem 12 is operable to cause the vehicle to travel in accordance witha value of set-speed in the range from 2-30 kph while the cruise controlsystem 16 is operable to cause the vehicle to travel in accordance witha value of set-speed in the range from 25-150 kph although other valuesare also useful. If the LSP control system 12 is selected when thevehicle speed is above 30 kph but less than or substantially equal to 50kph, the LSP control system 12 assumes the intermediate mode. In theintermediate mode, if the driver releases the accelerator pedal 161while travelling above 30 kph the LSP control system 12 deploys thebraking system 22 to gently slow the vehicle 100 to a value of set-speedcorresponding to the value of parameter LSP_set-speed. Once the vehiclespeed falls to 30 kph or below, the LSP control system 12 assumes theactive condition in which it is operable to apply positive drive torquevia the powertrain 129, as well as brake torque via the powertrain 129(via engine braking) and the braking system 22 in order to control thevehicle in accordance with the LSP_set-speed value. If the LSP controlsystem 12 is selected and no LSP set-speed value has been set, the LSPcontrol system 12 assumes the standby mode, the system 12 becomingactive once the ‘set +’ button 174 is depressed. In some embodiments, ifthe LSP control system 12 is selected when the vehicle speed is above 30kph but less than or substantially equal to 50 kph, the system 12deploys the braking system 22 to slow the vehicle 100 to 30 kph andprevents vehicle speed from exceeding 30 kph unless the driverover-rides the system 12 by depressing the accelerator pedal 161 orswitching off the system 12.

It is to be understood that if the LSP control system 12 is in theactive mode, operation of the cruise control system 16 is inhibited. Thetwo systems 12, 16 therefore operate independently of one another, sothat only one can be operable at any one time, depending on the speed atwhich the vehicle is travelling.

In the present embodiment, as noted above the cruise control HMI 18 andthe LSP control HMI 20 are configured within the same hardware so thatthe speed selection is input via the same hardware.

FIG. 4 illustrates the means by which vehicle speed is controlled whenthe LSP control system 12 is in the active mode. When in the active modethe LSP control system determines the amount of positive drive torque tobe applied by the powertrain 129, LSP_PT_TQ, and causes the powertrain129 to deliver this amount of torque by communicating the value ofLSP_PT_TQ to the powertrain controller 11. The value of LSP_PT_TQ may becommunicated to the powertrain controller 11 via the TC function block,which may arbitrate the value of LSP_PT_TQ in dependence on the amountof slip experienced by a driving wheel. Thus, the TC function block mayreduce the value of LSP_PT_TQ output to the powertrain controller 11when excessive slip is experienced.

When the LSP control system 12 is active, the amount of brake torque tobe applied by the braking system 22, LSP_BRK_TQ, is determined by theHDC control system 12HD, which is effectively ‘slaved’ to the LSPcontrol system 12 when the LSP control system 12 is active. The HDCsystem 12HD causes the braking system 22 to deliver this amount of braketorque by communicating the value of LSP_BRK_TQ to the ABS controller13. It is to be understood that the LSP control system 12 may cause theHDC control system 12HD to command a non-zero value of LSP_BRK_TQ whilethe LSP control system 12 is commanding application of positive (ornegative) powertrain torque, LSP_PT_TQ, in an automated implementationof ‘two pedaling’ where both brake and accelerator pedals are depressedby a driver to reduce wheel slip.

As shown in FIG. 4, the LSP control system 12 has an input functionblock 12 a that receives the following signals: a signal HDC_buttonindicating whether HDC system selector button 177 is currently pressed;a signal set_plus indicating whether the ‘set +’ button 174 is currentlypressed; and a signal Resume_button indicating whether the resume button173R is currently pressed.

In the embodiment of FIG. 4, the LSP control system 12 is configured tobecome active and command application of positive powertrain torque asrequired if the HDC selector button 177 is pressed for less than threeseconds while the LSP control system is not active and the ‘set +’button is subsequently pressed within 3 seconds of release of the HDCselector button 177. Other time periods are also useful.

The LSP control system input function block 12 a is arranged tocommunicate with a corresponding input function block 12HDa of the HDCcontrol system 12HD. If the LSP control system assumes the active mode,the LSP control system input function block 12 a provides a signalLSP_active to the HDC system 12HD signaling that the LSP control system12 is in the active state. With the LSP control system 12 in the activestate, the HDC system 12HD is configured to set the value ofHDC_set-speed to the value of LSP_set-speed and to operate in a slavemode to the LSP control system 12. That is, the HDC control system 12HDis operable to command application of brake torque by the ABS controller13 when commanded to do so by the LSP control system 12.

If neither the LSP control system 12 nor the HDC system 12HD are activeand the HDC selector button 177 is pressed for 3 s or longer, the HDCsystem 12HD becomes active. Under such circumstances the HDC system 12HDis not slaved to the LSP control system 12 and the LSP control system 12remains inactive.

If either the LSP control system 12 or the HDC system 12HD is active andthe HDC selector button is pressed for less than 3 s, the active system12, 12HD is deactivated.

As noted above, the HDC system 12HD is operable to apply brake torque toprevent vehicle speed exceeding HDC_set-speed (which is set equal toLSP_set-speed when the LSP control system is active), but not to applypositive powertrain torque.

The HDC control system input function block 12HDa is configured tooutput a value of LSP_set-speed to a target speed trajectory profilefunction block 12 b of the LSP control system 12 as well as to a targetspeed trajectory profile function block 12HDb of the HDC control system12HD. If the LSP control system 12 is activated with the vehiclesubstantially stationary, the value of LSP_set-speed is set to theminimum value at which the LSP control system 12 may cause a vehicle 100to operate. In the present embodiment this speed is substantially 2 kph.Other speeds may be set instead of 2 kph.

If the LSP control system 12 is activated while the vehicle 100 ismoving, the value of LSP_set-speed may be set to the instant vehiclespeed, v_actual as determined by the VCU 10.

Function block 12 b also receives as an input a signal TR_modeindicative of the driving mode (or ‘TR mode’) in which the vehicle 100is currently operating, and signal v_actual, indicating the speed of thevehicle 100 over ground as determined by the VCU 10.

The function block 12 b is configured to determine a target instantspeed value LSP_V_T and a target acceleration value LSP_A_T being,respectively, an instant speed at which the vehicle 100 is required totravel and an instant rate at which the vehicle is required toaccelerate to the instant speed, respectively. The function block 12 breceives as inputs the values of LSP_set-speed, TR_mode and v_actual.The value of each of these parameters is input to a look-up table whichgenerates the values of LSP_V_T and LSP_A_T. The values of theparameters LSP_V_T and LSP_A_T are input to a PI (proportional-integral)control module 12 c to generate a value of LSP_PT_TQ that is output tothe powertrain controller 129. Function block 12 b controls the value ofLSP_V_T and the value of LSP_A_T such that the target speed graduallybecomes equal to LSP_set-speed according to target speed trajectoryprofiles stored in a memory thereof.

The PI control module 12 c also receives as an input a valuecorresponding to the instant value of torque, PT_trq, being generated bythe powertrain 129, a value of a parameter A_actual corresponding to theactual instant rate of acceleration of the vehicle 100, the signalTR_mode and a value of a parameter ‘slope’ corresponding to a steepnessof a slope on which the vehicle 100 is driving. It is to be understoodthat A_actual may be positive or negative depending on whether thevehicle 100 is accelerating or decelerating. The value of ‘slope’ may bepositive or negative depending on whether the vehicle 100 is ascendingor descending a slope.

It is to be understood that in the present embodiment the values ofproportional feedback gain and integral feedback gain are adjusted independence on the TR mode in which the vehicle 100 is operating, asdetermined by reference to parameter TR_mode, and the driving surfacegradient, as determined by reference to parameter slope.

It is to be understood therefore, that the data stored in the look-uptable associated with function block 12 b is able to take account ofdifferences in the optimum rates of acceleration and deceleration forthe different TR modes. Function block 12 c, in turn, adjusts theserates in dependence on the gradient of the driving surface according tostored data. The manner in which the rates are adjusted in dependence onslope is further dependent on the TR mode, and therefore the functionblock 12 c receives the signals indicative of TR mode and drivingsurface gradient.

In the present embodiment the values are adjusted such that when thevehicle is in the ‘Sand’ TR mode and ascending a slope, the rate atwhich the value of LSP_PT_TQ increases when an increase in powertraintorque is required, due to target speed undershoot, is greater (i.e.more aggressive) than that when the vehicle is traversing level ground.When the vehicle is in the ‘Sand’ TR mode and ascending a slope and adecrease in powertrain torque is required, due to target speedovershoot, the rate at which LSP_PT_TQ decreases is lower (i.e. lessaggressive) than in the case where the vehicle is traversing levelground. This is because gravity is acting in favor of reducing vehiclespeed even in the absence of brake torque from the braking system 22,such that vehicle speed will reduce at a greater rate than if thevehicle 100 were travelling over level ground.

In contrast, if the vehicle 100 is in the GGS or the MR TR mode andascending a slope (i.e. over a driving surface with a positivegradient), the values of proportional feedback gain and integralfeedback gain are adjusted such that the rate at which the amount ofcommanded powertrain torque increases, when an increase in powertraintorque is required due to target speed undershoot, is lower (i.e. lessaggressive) than that when the vehicle is traversing level ground. Thisreduces the risk of excessive wheel slip, and therefore loss oftraction.

When the vehicle 100 is in the GGS or the MR TR mode and ascending aslope and a decrease in powertrain torque is required, due to targetspeed overshoot, the rate at which LSP_PT_TQ decreases is lower (i.e.less aggressive) than in the case where the vehicle is traversing levelground. This is at least in part because gravity is acting in favor ofreducing vehicle speed even in the absence of brake torque from thebraking system 22, such that vehicle speed will reduce at a greater ratethan if the vehicle 100 were travelling over level ground.

FIG. 6 illustrates schematically the general form of three gain profilesA, B, C corresponding to data stored by function block 12 b for use whenoperating in the sand TR mode when actual vehicle speed falls below thetarget speed, i.e. target speed undershoot occurs. The gain profiles areemployed by the function block 12 c to control the rate of increase ofthe amount of tractive torque that the powertrain controller 11 isrequested to deliver.

Profile B is a baseline profile and is used when travelling over asubstantially horizontal surface and target speed undershoot occurs.Profile A is used when travelling uphill and target speed undershootoccurs; it can be seen that the value of gain employed as a function ofspeed can be seen to be higher when travelling uphill compared to whentravelling over a substantially horizontal surface. That is, thefunction block 12 c is caused to command a more aggressive increase inpowertrain torque when undershoot occurs when travelling uphill,compared with travel over a substantially horizontal surface.

Profile C is used when travelling downhill and target speed undershootoccurs. It can be seen that the value of gain employed as a function ofspeed is lower when travelling downhill compared to when travelling overa substantially horizontal surface. That is, the function block 12 c iscaused to command a less aggressive increase in powertrain torque whenundershoot occurs when travelling downhill, compared with travel over asubstantially horizontal surface. This is due at least in part to thefact that gravity will tend to assist in the correction undershoot inthe case of downhill driving while gravity will tend to opposecorrection of undershoot when travelling uphill.

FIG. 7 illustrates schematically the general form of three gain profilesA, B, C corresponding to data stored by function block 12 b for use whenoperating in the GGS mode when actual vehicle speed falls below thetarget speed, i.e. target speed undershoot occurs. The gain profiles areemployed by the function block 12 c to increase the amount of tractivetorque that the powertrain controller 11 is requested to deliver.

Profile B is a baseline profile and is used when travelling over asubstantially horizontal surface and target speed undershoot occurs.Profile A is used when travelling uphill and target speed undershootoccurs; it can be seen that the value of gain employed as a function ofspeed can be seen to be lower when travelling uphill compared to whentravelling over a substantially horizontal surface. That is, thefunction block 12 c is caused to command a less aggressive increase inpowertrain torque when undershoot occurs when travelling uphill,compared with travel over a substantially horizontal surface. This hasthe advantage that the wheels are less likely to experience excessiveslip, increasing the risk that the vehicle fails to make adequateprogress over terrain, and/or the risk that undesirable modification ofthe driving surface occurs due to wheel slip. Surface modification istypically less problematic in the case of driving over sand.

Profile C is used when travelling downhill and target speed undershootoccurs. It can be seen that the value of gain employed as a function ofspeed is lower when travelling downhill compared to when travelling overa substantially horizontal surface, in a similar manner to the case whentravelling uphill in the GGS mode. That is, the function block 12 c iscaused to command a less aggressive increase in powertrain torque whenundershoot occurs when travelling downhill, compared with travel over asubstantially horizontal surface.

In the embodiment illustrated in FIG. 7 the value of gain is lower, fortravel downhill over a driving surface having a given value of slope,compared with the gain value for travel uphill over a driving surfacehaving the same value of slope as shown by the fact that profile C isbelow profile A in the figure. However, the value of gain for traveldownhill may be higher than, or substantially the same as, the value fortravel uphill, in some alternative embodiments.

If the vehicle is operating in the MR mode, the general relative form ofthe gain profiles for speed undershoot when travelling uphill, oversubstantially horizontal ground, or downhill, have a similarrelationship to that illustrated in FIG. 7, i.e. the gain valuesdecrease when travelling uphill or downhill compared to travel oversubstantially level ground.

In order to prevent or at least reduce passenger discomfort due to rapidchanges in acceleration rate (jerk), the LSP control system 12 limitsthe rate of change of acceleration of the vehicle 100, LSP_A_T, suchthat it does not exceed a prescribed maximum value. The value of LSP_A_Tis set in dependence on TR mode, the value for TR_mode=sand being higherthan the value for TR_mode=SPO, GGS or MR due to the higher drag imposedon a vehicle 100 traversing sand compared with a vehicle traversing adry asphalt highway surface, a grass, gravel or snow surface, or a muddyor rutted surface.

Furthermore, the value of LSP_A_T is controlled such that a steady staterate of acceleration is established the value of which is determinedaccording to the value of TR_mode. The steady state rate of accelerationis higher for high-drag surfaces such as sand compared with lower dragsurfaces in order to reduce a risk that a vehicle becomes stuck, i.e.unable to make adequate progress across terrain.

Turning to the HDC control system 12HD, the system 12HD has a functionblock 12HDb similar to the function block 12 b of the LSP control system12 that also receives signals TR_mode, v_actual and A_actual. Functionblock 12HDb is configured to determine, by reference to a look-up table,an instant value of a parameter HDC_V_T and parameter HDC_A_T based onthe signals TR_mode, v_actual and A_actual, and to output the value ofparameters HDC_V_T and HDC_A_T to a PI control module 12HDc. The valueof parameter HDC_V_T corresponds to a required target instant speed ofthe vehicle 100 and the value of parameter HDC_A_T corresponds to atarget instant rate of deceleration of the vehicle 100. Function block12HDb controls the value of HDC_V_T and the value of HDC_A_T such thatthe target speed gradually becomes equal to HDC_set-speed according totrajectory profiles stored in a memory thereof.

The value of HDC_A_T is controlled such that a maximum allowable rate ofchange of acceleration of the vehicle (referred to as a maximum jerkvalue) is not exceeded, the maximum allowed value of HDC_A_T whenTR_mode=sand being lower than that when TR_mode=SPO, GGS or MR due tothe more rapid deceleration of the vehicle when travelling over highdrag terrain such as sand compared with lower drag terrain, when theamount of drive torque to a wheel is reduced, due to the increased drag.Furthermore, the value of HDC_A_T is controlled such that a steady staterate of deceleration is established the value of which is determinedaccording to the value of TR_mode. The steady state rate of decelerationis arranged to be lower for high-drag surfaces such as sand comparedwith low-drag asphalt surfaces in order to reduce a risk that sanddisplaced by a wheel builds up in front of a wheel and causes abruptdeceleration. Abrupt deceleration typically compromises vehiclecomposure and is therefore typically undesirable.

The values of HDC_A_T and HDC_V_T are input to a PI(proportional-integral) control module 12HDc which generates a value ofHDC_BRK_TQ that is output to the ABS controller 13.

The PI control module 12HDc also receives as an input a valuecorresponding to the instant value of brake torque, BRK_trq, beinggenerated by the braking system 22, along with values of A_actual,‘slope’ and TR_mode. It is to be understood that the value of A_actualmay be positive or negative depending on whether the vehicle 100 isaccelerating or decelerating. The value of ‘slope’ is used to adjust avalue of proportional feedback gain and integral feedback gain of the PIcontrol module 12HDc according to the slope of the driving surface andthe TR_mode in which the vehicle 100 is driving. Thus, the functionblock 12HDc adjusts the values of proportional and integral feedbackgain constants employed by PI control module 12 c in dependence on thegradient of the driving surface and TR mode.

As noted above, when the LSP control system 12 is active, the HDCcontrol system 12HD is slaved to the LSP control system 12 and isconfigured to apply brake torque to the wheels as required. The LSPcontrol system 12 is configured to command less aggressive applicationof brake torque by the HDC control system 12HD when the vehicle isoperating in the sand mode compared to the SPO mode in order to reducethe risk that one or more wheels sink into the relatively compliantsurface and cause a relatively abrupt and substantial increase inresistance to vehicle movement. In the case of operation in the GGS orMR TR modes,

When the vehicle 100 is travelling uphill in one of the GGS or MR TRmodes and target speed overshoot occurs, the LSP control system 12 isconfigured to cause the PI control module 12HDc of the HDC controlsystem 12HD to operate with reduced gain values in order to reducevehicle speed (in the event braking is required) due to the effect ofgravity in assisting deceleration of the vehicle 100.

In contrast, when the vehicle is travelling downhill in one of the GGSor MR TR modes and target speed overshoot occurs, the LSP control system12 is configured to cause the PI control module 12HDc of the HDC controlsystem 12HD to operate with increased gain values relative to thoseemployed when travelling over substantially horizontal terrain. Thisresults in more aggressive braking when travelling downhill, and assistsin preventing excessive target overshoot due to the effect of gravity inresisting deceleration of the vehicle 100. However, the gains are notset to excessively high values, in order to reduce the risk of excessivewheel slip due to the relatively low surface coefficient of friction ofthe surfaces for which the GGS and MR TR modes are optimized, relativeto dry tarmac surfaces for which the SPO mode is optimized.

It is to be understood that in some embodiments in which a powertrain129 has one or more electric machines operable as a generator, negativetorque may be applied by the powertrain 129 to one or more wheels by theone or more electric machines. Negative torque may also be applied bymeans of engine braking in some circumstances, depending at least inpart on the speed at which the vehicle 100 is moving. If one or moreelectric machines are provided that are operable as propulsion motors,positive drive torque may be applied by means of the one or moreelectric machines when positive drive torque is commanded by the driveror LSP control system 12.

In order to cause application of the necessary positive or negativetorque to the wheels, the VCU 10 may command that positive or negativetorque is applied to the vehicle wheels by the powertrain 129 and/orthat a braking force is applied to the vehicle wheels by the brakingsystem 22, either or both of which may be used to implement the changein torque that is necessary to attain and maintain a required vehiclespeed. In some embodiments torque is applied to the vehicle wheelsindividually, for example by powertrain torque vectoring, so as tomaintain the vehicle at the required speed. Alternatively, in someembodiments torque may be applied to the wheels collectively to maintainthe required speed, for example in vehicles having drivelines wheretorque vectoring is not possible. In some embodiments, the powertraincontroller 11 may be operable to implement torque vectoring to controlan amount of torque applied to one or more wheels by controlling adriveline component such as a rear drive unit, front drive unit,differential or any other suitable component. For example, one or morecomponents of the driveline 130 may include one or more clutchesoperable to allow an amount of torque applied to one or more wheels tobe varied. Other arrangements may also be useful.

Where a powertrain 129 includes one or more electric machines, forexample one or more propulsion motors and/or generators, the powertraincontroller 11 may be operable to modulate torque applied to one or morewheels in order to implement torque vectoring by means of one or moreelectric machines.

In some embodiments the LSP control system 12 may receive a signalwheel_slip (also labelled 48 in FIG. 3 and FIG. 4) indicative of a wheelslip event having occurred. This signal 48 is also supplied to theon-highway cruise control system 16 of the vehicle, and which in thecase of the latter triggers an override or inhibit mode of operation inthe on-highway cruise control system 16 so that automatic control ofvehicle speed by the on-highway cruise control system 16 is suspended orcancelled. However, the LSP control system 12 is not arranged to cancelor suspend operation on receipt of wheel_slip signal 48. Rather, thesystem 12 is arranged to monitor and subsequently manage wheel slip soas to reduce driver workload. During a slip event, the LSP controlsystem 12 continues to compare the measured vehicle speed with the valueof LSP_set-speed, and continues to control automatically the torqueapplied to the vehicle wheels (by the powertrain 129 and braking system22) so as to maintain vehicle speed at the selected value. It is to beunderstood therefore that the LSP control system 12 is configureddifferently to the cruise control system 16, for which a wheel slipevent has the effect of overriding the cruise control function so thatmanual operation of the vehicle must be resumed, or speed control by thecruise control system 16 resumed by pressing the resume button 173R orset-speed button 173.

In a further embodiment of the present invention (not shown) a wheelslip signal 48 is derived not just from a comparison of wheel speeds,but further refined using sensor data indicative of the vehicle's speedover ground. Such a speed over ground determination may be made viaglobal positioning (GPS) data, or via a vehicle mounted radar or laserbased system arranged to determine the relative movement of the vehicle100 and the ground over which it is travelling. A camera system may beemployed for determining speed over ground in some embodiments.

At any stage of the LSP control process the user can override thefunction by depressing the accelerator pedal 161 and/or brake pedal 163to adjust the vehicle speed in a positive or negative sense. However, inthe event that a wheel slip event is detected via signal 48, the LSPcontrol system 12 remains active and control of vehicle speed by the LSPcontrol system 12 is not suspended. As shown in FIG. 4, this may beimplemented by providing a wheel slip event signal 48 to the LSP controlsystem 12, wheel slip then being managed by the LSP control system 12.In the present embodiment the SCS 14 generates the wheel slip eventsignal 48 and supplies it to the LSP control system 12 and cruisecontrol system 16. In some embodiments the ABS controller 13 generatesthe wheel slip event signal 48. Other arrangements may be useful.

A wheel slip event is triggered when a loss of traction occurs at anyone of the vehicle wheels. Wheels and tires may be more prone to losingtraction when travelling for example on snow, ice, mud or sand and/or onsteep gradients or cross-slopes. A vehicle 100 may also be more prone tolosing traction in other environments where the terrain is more unevenor slippery compared with driving on a highway in normal on-roadconditions. Embodiments of the present invention therefore findparticular benefit when the vehicle 100 is being driven in an off-roadenvironment, or in conditions in which wheel slip may commonly occur.Manual operation in such conditions can be a difficult and oftenstressful experience for the driver and may result in an uncomfortableride.

The vehicle 100 is also provided with additional sensors (not shown)which are representative of a variety of different parameters associatedwith vehicle motion and status. These may be inertial systems unique tothe LSP or HDC control systems 12, 12HD or part of an occupant restraintsystem or any other sub-system which may provide data from sensors suchas gyros and/or accelerometers that may be indicative of vehicle bodymovement and may provide a useful input to the LSP and/or HDC controlsystems 12, 12HD. The signals from the sensors provide, or are used tocalculate, a plurality of driving condition indicators (also referred toas terrain indicators) which are indicative of the nature of the terrainconditions over which the vehicle 100 is travelling.

The sensors (not shown) on the vehicle 100 include, but are not limitedto, sensors which provide continuous sensor outputs to the VCU 10,including wheel speed sensors, as mentioned previously, an ambienttemperature sensor, an atmospheric pressure sensor, tire pressuresensors, wheel articulation sensors, gyroscopic sensors to detectvehicular yaw, roll and pitch angle and rate, a vehicle speed sensor, alongitudinal acceleration sensor, an engine torque sensor (or enginetorque estimator), a steering angle sensor, a steering wheel speedsensor, a gradient sensor (or gradient estimator), a lateralacceleration sensor which may be part of the SCS 14, a brake pedalposition sensor, a brake pressure sensor, an accelerator pedal positionsensor, longitudinal, lateral and vertical motion sensors, and waterdetection sensors forming part of a vehicle wading assistance system(not shown). In other embodiments, only a selection of theaforementioned sensors may be used.

The VCU 10 also receives a signal from the steering controller 170C. Thesteering controller 170C is in the form of an electronic power assistedsteering unit (ePAS unit) 170C. The steering controller 170C provides asignal to the VCU 10 indicative of the steering force being applied tosteerable road wheels 111, 112 of the vehicle 100. This forcecorresponds to that applied by a user to the steering wheel 171 incombination with steering force generated by the ePAS unit 170C.

The VCU 10 evaluates the various sensor inputs to determine theprobability that each of a plurality of different control modes (drivingmodes) for the vehicle subsystems is appropriate, with each control modecorresponding to a particular terrain type over which the vehicle istravelling (for example, mud and ruts, sand, grass/gravel/snow).

If the user has selected operation of the vehicle in the automaticdriving mode selection condition, the VCU 10 then selects the mostappropriate one of the control modes and is configured automatically tocontrol the subsystems according to the selected mode. This aspect ofthe invention is described in further detail in our co-pending patentapplication nos. GB1111288.5, GB1211910.3 and GB1202427.9, the contentsof each of which is incorporated herein by reference.

As indicated above, the nature of the terrain over which the vehicle istravelling (as determined by reference to the selected control mode) mayalso be utilized in the LSP control system 12 to determine anappropriate increase or decrease in vehicle speed. For example, if theuser selects a value of user set-speed that is not suitable for thenature of the terrain over which the vehicle is travelling, the system12 is operable to automatically adjust the vehicle speed downwards byreducing the speed of the vehicle wheels. In some cases, for example,the user selected speed may not be achievable or appropriate overcertain terrain types, particularly in the case of uneven or roughsurfaces. If the system 12 selects a set-speed (a value ofLSP_set-speed) that differs from the user-selected set-speeduser_set-speed, a visual indication of the speed constraint is providedto the user via the LSP HMI 20 to indicate that an alternative speed hasbeen adopted.

It is to be understood that, when driving downhill on sand, it may bedesirable not to apply negative torque to wheels of the vehicle 100. Asdescribed above, this is because the wheels will have a tendency to diginto the sand, the effect being enhanced by the nose-down, weightforward condition during vehicle descent. This may be achieved byrelaxing the rate at which negative torque is applied by a brakingsystem 22, in the present embodiment by reducing the proportional andintegral feedback gain values of the PI control module 12HDc.

In some embodiments, the VCU 10 may be configured such that the LSPcontrol system 12 tends to allow the value of v_actual to increase tobecome substantially equal to LSP_set-speed by coasting rather than byapplying positive powertrain torque. in order to achieve this, in thepresent embodiment the proportional and integral feedback gain values ofthe PI control module 12 c are set to relatively low values when thevalue of ‘slope’ indicates a downhill slope. The actual proportional andintegral feedback gain values may in some embodiments be arranged tobecome progressively higher as the value of ‘slope’ indicates anincreasingly steep downhill slope. In some embodiments the actualproportional and integral feedback gain values are set to sufficientlylow values that they substantially prevent application of positivetorque as the vehicle accelerates downhill towards LSP_set-speed.

It is to be understood that, in some embodiments, in addition toproviding a signal TR_mode to the function blocks 12 b, 12HDb, aparameter indicative of an actual amount of drag on a vehicle due toexternal forces may be provided. The function blocks 12 b, 12HDb may bearranged to determine, respectively, the values of LSP_V_T, LSP_A_T andHDC_V_T, HDC_A_T in dependence on the amount of drag as well as orinstead of the selected TR mode. It is to be understood that travel oversand corresponds to travel over terrain for which the amount of externaldrag is relatively high. Means for measuring external drag forces on avehicle are well known.

In some situations, a vehicle 100 may descend an incline at a speedbelow LSP_set-speed and the LSP control system 12 may be required tocause application of positive powertrain drive torque to accelerate thevehicle 100 to LSP_set-speed. In such circumstances, in some embodimentsfunction blocks 12 b, 12 c may be configured to set the value ofLSP_PT_TQ to a value corresponding to substantially no positivepowertrain drive torque prior to v_actual attaining LSP_set-speed. Thisis so as to prevent excessive overshoot of LSP_set-speed by v_actual,and be performed in dependence on the value of ‘slope’ and TR mode. Thisprocedure may enable the vehicle 100 to descend the slope without arequirement to apply brake torque to one or more wheels. Application ofbrake torque may give rise to sudden, undesirably high deceleration anddegrade vehicle composure. It is to be understood that the LSP controlsystem 12 may take advantage of a drag force on the vehicle 100 due tothe high drag terrain to mitigate excessive over-speed as the vehicledescends the slope. Should excessive overshoot occur, the HDC controlsystem 12HD may be arranged to cause application of brake torque in amore gentle manner (by appropriate control of the values of HDC_V_T andHDC_A_T).

Other arrangements may be useful.

Some embodiments of the present invention enable vehicle operation withenhanced composure on driving surfaces of different gradients.

In addition, some embodiments of the present invention have theadvantage that sudden over-braking on high drag terrain such as sand maybe prevented. Some embodiments of the present invention give rise togreatly enhanced vehicle composure when driving across varied terrain,especially over high drag, deformable surfaces such as sand.

It will be understood that the embodiments described above are given byway of example only and are not intended to limit the invention, thescope of which is defined in the appended claims.

Throughout the description and claims of this specification, the words“comprise” and “contain” and variations of the words, for example“comprising” and “comprises”, means “including but not limited to”, andis not intended to (and does not) exclude other moieties, additives,components, integers or steps.

Throughout the description and claims of this specification, thesingular encompasses the plural unless the context otherwise requires.In particular, where the indefinite article is used, the specificationis to be understood as contemplating plurality as well as singularity,unless the context requires otherwise.

Features, integers, characteristics, compounds, chemical moieties orgroups described in conjunction with a particular aspect, embodiment orexample of the invention are to be understood to be applicable to anyother aspect, embodiment or example described herein unless incompatibletherewith.

The reader's attention is directed to all papers and documents which arefiled concurrently with or previous to this specification in connectionwith this application and which are open to public inspection with thisspecification, and the contents of all such papers and documents areincorporated herein by reference.

1. A speed control system for a vehicle, comprising: torque controlmeans for automatically causing application of positive and negativetorque to one or more wheels of a vehicle to cause a vehicle to travelin accordance with a target speed value; and means for receivinginformation indicative of a set speed, and information indicative of aterrain response mode, the torque control means being configured todefine the target speed value in accordance with the informationindicative of a set speed and the information indicative of a terrainresponse mode, the speed control system further comprising means forreceiving information indicative of a gradient of a driving surface overwhich the vehicle is driving, the torque control means being configuredto control the rate of change of the amount of torque applied to the oneor more wheels in accordance with the target speed value, the rate ofchange of the amount of torque being adjusted in dependence at least inpart on the information indicative of a terrain response mode and theinformation indicative of a gradient of the driving surface.
 2. A systemaccording to claim 1 wherein the torque control means is configured toattempt to cause the vehicle to travel at a speed substantially equal tothe target speed, the torque control means being configured to controlthe rate of change of the amount of torque applied to the one or morewheels, in order to attempt to maintain the vehicle travelingsubstantially at the target speed value, in dependence at least in parton the gradient of the driving surface.
 3. A control system according toclaim 1 wherein the torque control means is configured wherein whenactual vehicle speed is less than the target speed value and theinformation indicative of surface gradient indicates the vehicle istraveling uphill, the torque control means attempts to cause the vehicleto accelerate towards the target speed value at a rate that is higherthan when driving on a substantially horizontal surface.
 4. A controlsystem according to claim 3 wherein the torque control means isconfigured wherein when actual vehicle speed is less than the targetspeed value, the torque control means attempts to cause the vehicle toaccelerate towards the target speed value at a rate that isprogressively higher for progressively higher values of uphill drivingsurface gradient.
 5. A control system according to claim 1 wherein thetorque control means is configured wherein when actual vehicle speed isless than the target speed value and the information indicative ofsurface gradient indicates the vehicle is traveling uphill, the torquecontrol means attempts to cause the vehicle to accelerate towards thetarget speed value at a rate that is lower than when driving on asubstantially horizontal surface.
 6. A control system according to claim5 wherein the torque control means is configured wherein when actualvehicle speed is less than the target speed value, the torque controlmeans attempts to cause the vehicle to accelerate towards the targetspeed value at a rate that is increasingly lower for increasingly highervalues of uphill driving surface gradient.
 7. A control system accordingto claim 2 wherein the torque control means is configured wherein whenactual vehicle speed is greater than the target speed value, the torquecontrol means causes a reduction in torque applied to the one or morewheels in order to attempt to cause the vehicle to travel at a speedsubstantially equal to the target speed at a rate that is lower for agiven deviation in speed above the target speed than in the case of acorresponding deviation below the target speed value.
 8. A controlsystem according to claim 7 wherein the torque control means isconfigured wherein when actual vehicle speed is greater than the targetspeed value and the vehicle is traveling uphill, the torque controlmeans causes a reduction in torque applied to the one or more wheels inorder to attempt to cause the vehicle to travel at a speed substantiallyequal to the target speed at a rate that is substantially equal to orlower than in the case of a corresponding deviation in vehicle speedabove the target speed whilst traveling over a substantially horizontaldriving surface.
 9. A control system according to claim 2 wherein thetorque control means is configured wherein when actual vehicle speed isgreater than the target speed value and the vehicle is travellingdownhill, the torque control means causes a reduction in torque appliedto the one or more wheels in order to attempt to cause the vehicle totravel at a speed substantially equal to the target speed at a rate thatis substantially equal to or lower than in the case of a correspondingdeviation in vehicle speed above the target speed whilst traveling overa substantially horizontal driving surface.
 10. A control systemaccording to claim 2 wherein the torque control means is configuredwherein when actual vehicle speed is greater than the target speed valueand the vehicle is travelling downhill, the torque control means causesa reduction in torque applied to the one or more wheels, in order toattempt to cause the vehicle to travel at a speed substantially equal tothe target speed, at a rate that is greater than in the case of acorresponding deviation in vehicle speed above the target speed whilsttraveling over a substantially horizontal driving surface.
 11. A controlsystem according to claim 2 configured to cause the vehicle toaccelerate from a first speed to the target speed value, where the firstspeed is less than the target speed value, at least in part according tostored data in respect of target rate of acceleration as a function ofspeed, wherein the value of target rate of acceleration according towhich the vehicle is caused to accelerate is determined in furtherdependence at least in part on the driving surface gradient.
 12. Acontrol system according to claim 2 configured to cause a vehicle todecelerate from a second speed to the target speed value, where thesecond speed is greater than the target speed value, according to storeddata in respect of target rate of deceleration as a function of speed,wherein the target rate of deceleration according to which the vehicleis caused to decelerate is determined in further dependence at least inpart on the driving surface gradient.
 13. A control system according toclaim 2 configured to control a rate of change of vehicle speed towardsthe target speed iteratively by causing the vehicle to attempt toachieve an intermediate instant target speed, the value of intermediateinstant target speed and therefore vehicle speed being caused to changein an iterative manner towards the target speed value at a requiredrate.
 14. A control system according to claim 1 operable to control arate of change of vehicle speed so as not to exceed a prescribed jerkvalue wherein the prescribed jerk value is set in dependence on thegradient of the driving surface. 15-18. (canceled)
 19. A control systemaccording to claim 1 wherein the terrain response mode is one of aplurality of driving modes in which each one of a plurality of vehiclesubsystems is caused to operate in one of a plurality of configurationmodes of that subsystem, the subsystem configuration mode beingdetermined in dependence on the selected driving mode.
 20. A controlsystem according to claim 19 configured wherein the torque control meansis operable to control the rate of change of the amount of torqueapplied to the one or more wheels, in order to attempt to maintain thevehicle traveling in accordance with the target speed value, independence at least in part on the information indicative of drivingsurface gradient only if the vehicle is operating in a driving mode thatis a member of a predetermined group of one or more of the plurality ofdriving modes.
 21. A control system according to claim 20 configuredwherein when actual vehicle speed is less than the target speed valueand the information indicative of surface gradient indicates the vehicleis traveling uphill, the torque control means attempts to cause thevehicle to accelerate towards the target speed value at a rate that islower than when driving on a substantially horizontal surface if thevehicle is operating in a driving mode that is a member of a first groupof one or more of the driving modes and is not a driving mode that isnot a member of the first group.
 22. A control system according to claim21 wherein the first group of driving modes comprises at least onedriving mode adapted for driving on a driving surface of relatively lowsurface coefficient of friction, optionally wherein the first group ofdriving modes comprises at least one driving mode adapted for driving ona driving surface of relatively low surface coefficient of frictionexcluding a mode adapted for driving on sand, further optionally whereinthe first group of driving modes comprises at least one driving modeadapted for driving on at least one of a snowy surface, an icy surface,grass, gravel, snow and mud. 23-28. (canceled)
 29. A vehicle comprisinga speed control system according to claim
 1. 30. A method of controllinga vehicle implemented by means of a control system, comprising:automatically causing application of positive and negative torque to oneor more wheels of a vehicle to cause a vehicle to travel in accordancewith a target speed value; receiving information indicative of a setspeed, and information indicative of a terrain response mode, definingthe target speed value in accordance with the information indicative ofa set speed and the information indicative of a terrain response mode,receiving information indicative of a gradient of a driving surface overwhich the vehicle is driving, and controlling the rate of change of theamount of torque applied to the one or more wheels, in order to attemptto maintain the vehicle traveling in accordance with the target speedvalue, in dependence at least in part on the information indicative of aterrain response mode and the information indicative of a gradient ofthe driving surface. 31-35. (canceled)
 36. A non-transitorycomputer-readable storage medium storing instructions thereon that whenexecuted by one or more electronic processors causes the one or moreelectronic processors to carry out the method of claim 30.